The invention relates to apparatus for classifying streams or batches which contain mixtures of randomly distributed larger (particularly longer and harder) and smaller (particularly shorter and softer) particles. Typical examples of streams or batches (hereinafter referred to as streams) which can be treated in the apparatus of the present invention are those containing fragments of tobacco stem and/or ribs in random distribution with shreds of tobacco leaf laminae.
The so-called distributor (also known as hopper) of a cigarette rod making machine comprises a magazine for reception of a relatively large supply of tobacco particles. Such particles are a mixture of longer and heavier particles (particularly fragments of tobacco stem and/or ribs) and shorter and softer particles, such as shreaded tobacco leaf laminae. The distributor is equipped with one or more sifters which are designed to segregate heavier particles from lighter particles and to thus prevent the heavier particles from entering the wrapping station of the cigarette rod making machine wherein a continuous rod-like tobacco filler is draped into a web of thin and readily penetrable wrapping material, normally a web of cigarette paper. Any longer and relatively hard particles which enter the rod-like filler are likely to puncture or tear the wrapper so that the corresponding cigarettes must be segregated from satisfactory cigarettes (namely from cigarettes with wrappers which are devoid of holes) for introduction into a cigarette ripping or breaking apparatus which is used to destroy the wrappers and to thus permit recovery of tobacco particles for reintroduction into the distributor.
The making of the filler in a cigarette rod making machine is preceded by conversion of tobacco particles into a relatively wide carpet which is thereupon sifted (normally by pneumatic means) to segregate heavier and harder particles from softer and lighter particles, and the softer and lighter particles are thereupon showered against an air-permeable conveyor which builds a continuous stream containing a surplus of tobacco particles. The stream is trimmed to remove the surplus, and the thus obtained trimmed stream constitutes a filler which is ready to be draped into a web of cigarette paper or the like. The resulting cigarette rod (wrapped filler) is severed in a cutoff to yield plain cigarettes of unit length or multiple unit length. The cigarettes are delivered to a packing machine, to storage or to a filter tipping machine.
Though the sifters (as mentioned above, such sifters normally include means for pneumatically segregating lighter particles from heavier particles) in presently known distributors or hoppers are quite efficient, it happens again and again that a certain percentage of satisfactory particles (particularly shreds) is entrained with the heavier particles and is discarded or processed with the heavier particles (fragments of stem and/or ribs) instead of being admitted into the rod forming zone. For example, certain lighter particles are so intimately entangled with adjacent heavier particles that they cannot be segregated from heavier particles during advancement across a so-called curtain of compressed air which is used to intercept and deflect lighter particles but is too weak to overcome the inertia of the heavier particles. Thus, the heavier particles traverse the curtain and are gathered in a suitable receptacle which is evacuated, either continuously or at intervals, to provide room for admission of additional heavier particles. The percentage of lighter particles which are entrained with the heavier particles is, or is likely to be, sufficiently high to warrant their recovery and readmission into the distributor or directly into the rod forming zone.
German Utility Model No. G 90 00 291.1 of Niepmann Traylift Transportsysteme GmbH & Co. KG (registered Mar. 1, 1990) discloses an apparatus which is designed to segregate lighter and shorter particles from heavier and longer particles in a tobacco stream wherein the two types of particles are in random distribution. The apparatus which is described in the Utility Model comprises several vibratory trays which are disposed one above the other. The uppermost tray is located beneath the outlet of a duct which delivers a mixture of longer and shorter tobacco particles, and the bottom wall of the uppermost tray is a grating which is to permit passage of certain particles but should retain the other (larger) particles. The second topmost tray has a bottom wall with relatively short longitudinally extending slots which permit certain particles to descend into the third tray having relatively small circular openings for the passage of certain particles onto the non-apertured and unslotted bottom wall of a fourth tray.
A drawback of the just described apparatus is that particles which happen to be caught in the interstices, slots and/or openings in the bottom walls of the upper trays clog the respective bottom walls after a short period of use. This necessitates frequent inspection and cleaning of the bottom walls. Moreover, once the interstices, slots or openings of a bottom wall are partially or completely clogged, the classifying action of the apparatus is immediately affected with the result that heavier and longer particles are separated with the shorter and lighter particles and/or vice versa.